1. Field of the Invention
This invention relates to an image-forming apparatus such as image display apparatus and, more particularly, it relates to the configuration of the face plate thereof.
2. Related Background Art
Intensive technological efforts are being paid to realize an ever larger display screen for image-forming apparatus comprising a cathode ray tube or CRT. The efforts are generally directed to technological problems to be resolved in order to reduce the depth, the weight and the cost of the apparatus.
The inventor of the present invention has been engaged in technological research on multiple electron beam sources and image-forming apparatus using them that can be realized by arranging a large number of surface conduction electron-emitting devices particularly in terms of materials, manufacturing method and structure.
FIG. 16 of the accompanying drawings schematically illustrates a wiring arrangement applicable to a multiple electron beam source proposed by the inventors of the present invention. Such a multiple electron beam source comprises a large number of surface conduction electron-emitting devices arranged two-dimensionally and provided with a simple matrix wiring arrangement as shown.
Referring to FIG. 16, reference numeral 4001 denotes surface conduction electron-emitting devices that are illustrated only schematically and reference numeral 4002 denotes row-directional wires, whereas reference numeral 4003 denotes column-directional wires. Note that the illustrated matrix has only 6 rows and 6 columns for the purpose of simplicity and convenience, although the number of wires will be selected to make the apparatus display intended images.
FIG. 17 is a partially cut out schematic perspective view of a cathode ray tube realized by using such a multiple electron beam source and comprising an envelope bottom 4005 provided with a multiple electron beam source 4004, an envelope frame 4007 and a face plate 4006 having an fluorescent layer 4008 and a metal back 4009. A high voltage is applied to the metal back 4009 of the face plate 4006 from a high voltage source 4010 by way of a high voltage introducing terminal 4011.
In a multiple electron beam source comprising surface conduction electron-emitting devices and provided with a simple matrix wiring arrangement, electric signals are applied appropriately and selectively to the row-directional wires 4002 and the column-directional wires 4003 in order to make the devices emit electrons in a desired manner. For example, when the surface conduction electron-emitting devices of a selected row of the matrix are driven, a selection voltage Vs is applied to the row-directional wire 4002 of the selected row and a non-selection voltage Vns is applied to the row-directional wires 4002 of the unselected remaining rows simultaneously. Then, a drive voltage Ve is synchronously applied to the column-directional wires 4003 to make the selected devices emit electron beams. With this technique, a voltage of Ve-Vs is applied to all the surface conduction electron-emitting devices of the selected row, whereas a voltage of Ve-Vns is applied to all the surface conduction electron-emitting devices of the unselected rows. Thus, by selecting appropriate values for voltages Ve, Vs and Vns, only the surface conduction electron-emitting devices of the selected row emit electron beams with a desired intensity. The surface conduction electron-emitting devices of the selected row can be made to emit electron beams with different respective intensities by varying the drive voltage Ve for each column-directional wires. Since surface conduction electron-emitting devices respond very quickly, the time during which electron beams are emitted from the surface conduction electron-emitting devices can be controlled by controlling the time for applying the drive voltage Ve.
Then, electron beams emitted from the multiple electron beam source 4004 are made to irradiate the metal back 4009 to which a high voltage is applied and energize the fluorescers to emit light. Therefore, an image-forming apparatus comprising such a multiple electron beam source can be made to display desired images by applying appropriate voltage signals to it in a controlled manner.
In the above described image-forming apparatus, the face plate 4006, the bottom of the envelope 4005 and the frame of the envelope 4007 are typically made of soda lime glass because then the envelope can be assembled without any difficulty.
When a high voltage is applied to the inner surface of the face plate 4006, a light electric current can flow from the inner surface to the outer surface of the face plate due to the electric field generated between the inner surface and the electric potential of the ground GND surrounding the apparatus. This is the electric current that flows as sodium (Na) atoms in the soda lime glass of the face plate 4006 are positively ionized and move. As Na cations move and get to the outer surface of the face plate 4006, some of them are deposited on the surface to make the face plate 4006 show a coarse surface. Some of the deposited Na cations can react with moisture in the air to produce sodium hydroxide and make the surface opaque. Then, the face plate 4006 will lose its optical transmissivity and contrast to a significant extent to consequently degrade the quality of the images displayed on the display screen. The migration of Na ions can also degrade the withstand voltage of the face plate.
Additionally, as the electric potential of the outer surface of the face plate 4006 rises, dirt can adhere to the surface to also degrade the quality of the images displayed on the screen. The electric potential of the inner surface of the face plate can also be changed by the raised potential of the outer surface. The viewer or observer of the display screen can become a victim of electric discharges that can take place when he or she gets closer to the face plate.
According to a known technique to eliminate the above problem, a transparent anti-charge film 4012 is formed on the surface of the face plate 4006 and grounded as shown in FIG. 18 to prevent the electric potential of the surface of the face plate.
However, with a glass face plate 4006 provided with an anti-charge film 4012 formed on the surface and grounded, a large potential difference of Va is produced between the front surface and the rear surface of the face plate 4006 when the high voltage Va is applied to the metal back 4009 arranged on the rear surface of the face plate as target of cathode rays. If the face plate is made of soda lime glass containing Na to a large concentration, Na cations inside the glass can move and become deposited on the grounding electrode side or the side of the anti-charge film 4012 when the high voltage Va is applied for a prolonged period of time regardless of the provision of the anti-charge film 4012.
This problem may be avoided by selecting a glass plate having a thickness of several centimeters for the face plate to reduce the field strength and slow down the moving rate of Na cations or using glass containing Na to a very low concentration for the face plate. However, the use of a face plate as thick as several centimeters will make the image-forming apparatus comprising such a face plate very heavy while the use of glass containing little Na will be a costly choice.
An alternative technique for avoiding the problem may be the use of a protector plate made of resin that is lightweight relative to glass and arranged on the glass face plate to reduce the voltage applied to the face plate.
Therefore, an object of the present invention is to provide an image-forming apparatus such as image display apparatus that can display images without degradation with time of the quality of displayed images.
Another object of the invention is to provide an image-forming apparatus that can display images without degradation with time of the optical transmissivity of the image-forming side (face plate side) of the apparatus.
Still another object of the invention is to provide an image-forming apparatus that is lightweight and can be manufactured at low cost.
According to the invention, there is provided an image-forming apparatus comprising an envelope and an image-forming means having a member adapted to application of voltage Va, characterized in that the member carrying on the inner surface thereof said member adapted to application of voltage Va and constituting part of the envelope also carries a means for applying a voltage substantially equal to said voltage Va.